The invention concerns an apparatus for stretching out a cloth portion, in particular a rectangular gauze material, on a printing frame with stretching devices which are arranged at the sides of the frame and in which the cloth portion is fixed by means of a respective clamping mouth comprising clamping jaws which are movable relative to each other and which can be applied to each other on the closing travel of a closing lever under a pressing pressure, and the cloth portion is then tensioned by a change in position of the clamping mouth units, caused by a piston/cylinder unit for a flow medium, in the pulling direction.
An apparatus of that kind is described in Swiss patent specification No. 679 918, with the indication that the production of printed circuit boards by a screen printing procedure involves a very high level of accuracy in respect of the cloth portion which is used in a screen printing stencil, and for that reason extremely uniform stretching of the cloth portion is required; irregularities in the distribution of tension give rise to distortion phenomena in the cloth portion and stress and tension peaks which can occur therein can result in undesired tearing. According to Swiss patent specification No. 679 918 those disadvantages are said to be possible by virtue of a clamping mouth which is closable by means of a mechanical lever system, the clamping mouth comprising a pair of clamping jaws connected to the free end of a piston rod of the piston/cylinder unit by a universal joint. The fact that an upper clamping jaw is mechanically pressed by a closing lever on to a lower clamping jaw which is fixed in the pressing direction means that they cannot be individually acted upon by a quantifiable pressing force, in other words, it is not possible to provide for adaption thereof to the respective factors involved, when the cloth portion is clamped in place.